A new study found that UV light can help kill bacteria.

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Bacteria tend to grow more in darker spaces. Getty Images

New research shows that sunlight kills potentially harmful bacteria and improves air quality in indoor environments.

Researchers from the University of Oregon built 11 climate-controlled miniature rooms to reconstruct the atmospheres of homes or office buildings. The goal of the experiment was to study the effect that regular sunlight, ultraviolet (UV) light, and no light have on a room’s microbiome, or the mix of bacteria that exists in any environment.

The authors of the study published last month collected dust from homes and then seeded each of the dollhouse-sized rooms with this residential dust.

They left the rooms outside in sunlight while maintaining typical indoor temperatures inside the rooms for 90 days.

After the three months, the authors collected dust samples from each of the 11 rooms and looked at the amount and viability (the ability to reproduce) of the dust that remained.

In the rooms that had no light, 12 percent of bacteria remained alive and viable. In the rooms that were exposed to daylight, 6.8 percent of bacteria were viable — almost half that of the dark rooms.

Rooms exposed to UV light had slightly less live bacteria than the rooms exposed to sunlight; 6.1 percent of bacteria in these rooms were still viable 90 days after the start of the experiment.

Sunshine as medicine

The idea that sunshine is healthy, even in indoor environments, isn’t new. Architects and designers have long created spaces that allowed for light, both for aesthetic purposes as well as health ones. Doctors have even prescribed it as treatment.

“There are lots of grandmothers who knew sunlight was good for you, for many reasons,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University School of Medicine in Nashville, Tennessee.

“Back in the day before we had drugs that treated tuberculosis (TB), people with TB were put outside frequently to be out in the fresh air and the sunshine, because that was thought to promote health in a variety of ways,” he said.

Visible light can also help keep circadian rhythms in natural order. Sunlight can also help your body produce vitamin D. This nutrient is important for bone, brain, and heart health.

Thanks to this study, scientists now know a bit more about the effects of sunlight on the bacteria living inside the buildings and what it could mean for your health.

The findings, which were published recently in the journal Microbiome, could help builders understand how visible light affects real bacterial communities in the places where people spend the majority of their days.

The researchers were surprised by the similar bacterial losses in the rooms exposed to visible light (sunlight) and those exposed to UV light.

UV light vs. bacteria

UV light has been used for decades as a natural disinfectant. It’s used to clean drinking water as well as in some environments, like hospitals and medical facilities, to naturally eradicate organisms that could be potentially harmful.

The fact that sunlight was similarly effective as UV light was a surprise to the authors. That’s in part because most window glass filters out UV light, the element of sunlight that has some known beneficial, antibacterial effect.

“There have been past studies to indicate that sunlight and indeed ultraviolet light kills bacteria and viruses,” Schaffner said. “The major takeaway from this study is that even if the sunlight goes through conventional panes of glass, it retains a substantial capacity to kill bacteria and viruses. We did not know whether the sunlight passing through panes of glass would retain some of this capacity to disinfect, and it does. So that’s a great thing.”

What was also surprising to the researchers was the number of microbes that remained viable in dust.

Dust has long been considered too dry for robust bacterial growth. Though only 6 and 12 percent of bacteria remained in the study’s UV light and dark rooms, respectively, that’s still millions of cells.

Indoor bacteria resembled outdoor bacteria

Another surprising element for the study’s authors was the composition of the bacteria in the sunlit rooms.

These bacteria had a smaller proportion of human skin-derived bacteria compared to the bacteria in the dark rooms. Instead, there was a higher proportion of outdoor air-derived bacteria.

This suggests sun exposure may shape the microbiome of an indoor room to more strongly resemble the microbiome of the outdoors.

Additionally, some of the bacteria that were killed in the sunlit rooms but remained in the dark rooms are known to cause respiratory disease.

“Sunlight has UV light — UVA and UVB [individual wavelengths in the ultraviolet spectrum] — and they have germicidal capabilities,” said Luis Romero, founder and CEO of PurpleSun, a healthcare technology company that provides light-based solutions for infection prevention and medical equipment disinfection.

“You’ve heard that expression that in nature moss only grows on the north side of trees,” he said, citing a folklore expression that helps people know the direction they’re facing based on where moss grows. “Bacteria and mold grow where there is less sunlight, so sun exposure is going to have a germicidal effect.”

Ashkaan K. Fahimipour, PhD, the study’s lead author and a post-doctoral researcher in the University of Oregon’s Biology and Built Environment Center, says their study, while enlightening, does have some limits.

The miniature rooms they created for the experiment don’t reflect the varied size and architectural styles of offices and homes. These factors may influence bacteria composition and viability.

Likewise, these rooms remained shut off, no longer exposed to additional bacterial seeding that a real-world environment, like a home or office, would have with continual use.

“We need more research to understand the underlying causes of shifts in the dust microbiome following light exposure,” Fahimipour said in a statement. “We hope that with further understanding, we could design access to daylight in buildings such as schools, offices, hospitals and homes in ways that reduce the risk of dust-borne infections.”

The bottom line

Invite more natural sunlight into your house or office by opening blinds or pulling back curtains. Sunlight may have a bacteria-killing benefit as well as a naturally mood-enhancing one.

“We don’t need to open up our windows to let in the sunshine. We just need to make sure the shades are up or the blinds are set so that the sunlight can come in,” Schaffner said.

During the winter months, when you’re likely spending more time indoors and coming into contact with germs and viruses that could make you quite ill (the flu comes to mind), this boost of sunlight may help you feel better and prevent illness.